NMR of Surface Hydroxyl Groups in Loaded Samples

As described in Sect. 2.3, 3h and vqh are linearly correlated at least in limited ranges. It is, therefore, tempting to assiune a similar correlation between A5h and Avon given by  

The induced H NMR chemical shift A5h caused by the adsorption of hydrogen-bond forming probe molecules M on surface hydroxyl groups can be strongly reduced by rapid thermal motions and/or exchange processes of the probe molecules [95]. In most cases it is, therefore, necessary to carry out the corresponding H MAS NMR measurements at low temperatures. The influence of small and weakly basic probe molecules on the low-temperature H MAS NMR spectra of zeohtes was investigated for the first time by White et al. [96]. Table 2 summarizes induced chemical shift values for a variety of 

Loading is given in molecules (M) per framework A1 atom (Al ), and A3h in ppm. Measurements were performed at a temperature of 123 K 

Conditions (i) and (ii) are obviously fulfilled for CO and C2H4. However, it turned out that C2H4 chemically reacts rapidly at room temperature in H-zeolites, which prevents its use as a probe molecule. Two selected examples will be considered for the demonstration of characteristic phenomena caused by the adsorption of small and weakly basic probe molecules on zeolites, namely the adsorption of CO and C2CI4 (instead of C2H4). 

CO [36,91,92]. Furthermore, it is remarkable that the SiOH groups are completely shifted from 2.0 ppm to 2.8 ppm. 

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